The invention relates to a method and a device for initiating a hydrogen/oxygen reaction in a catalytic recombination or ignition device equipped with at least one catalyst body formed with a predetermined catalytically active surface.
Catalytic ignition devices are described, for example, in the European patent application EP 0 303 144 A1 and in the German patent application. DE 30 04 677 A1. A catalytic recombination device is known, for example, from the European patent application EP 0 527 968 A1.
After an incident involving a coolant loss in a nuclear power plant, large quantities of hydrogen H2 and carbon monoxide CO may be released into the safety vessel. Unless proper countermeasures are taken, the hydrogen enrichment in the atmosphere of the safety vessel containment would, under some circumstances, be possible to such an extent that an explosive mixture could be formed. In the event of late accidental ignition, the structural integrity of the safety vessel could be compromised and put at risk, above all due to the combustion of a relatively large quantity of hydrogen. Consequently, efforts are being made to eliminate the hydrogen and carbon monoxide from the atmosphere of the safety vessel by recombination with oxygen O2. The efforts are directed towards developing and optimizing a recombination or ignition device that starts early. Such a device should not lose its activity to any appreciable extent even in the event of a relatively long operating period in the atmosphere of the safety vessel. It should further be capable of starting completely passively at low ambient temperatures. In other words, it is to be possible to operate a catalytic recombination or ignition device passively and with a fast start even in the event of incidents at low ambient temperatures, for example in the case of TMI incidents with the ventilation being in operation and with resulting ambient temperatures of only 30xc2x0 C. or else under ice condenser conditions of, for example, 10xc2x0 C. and below. If it is possible for the hydrogen to be reduced passively in good time, the safety of the nuclear power plant is increased decisively. Conventional catalytic recombination or ignition devices work with catalyst elements based on platinum and/or palladium. It is the object to increase the reaction starting safety decisively even when reversible catalyst poisons are present.
A prior art recombination device for the recombination of hydrogen and oxygen is described in the European patent EP 0 527 986 C1. In one embodiment of that specification, a plurality of catalyst plates are arranged parallel to one another in a housing. The housing is formed with an inflow orifice and an outflow orifice, such that the gas/vapor mixture, which, during the incident, contains the hydrogen to be eliminated, flows in parallel through the mutually parallel catalyst plates. The device is intended, in particular, for use after serious incidents, in which temperatures of more than 50xc2x0 C. in the atmosphere of the safety vessel must be expected. It would be desirable to design such a device in such a way that it starts reliably even at lower temperatures.
The foregoing publication describes platinum and palladium catalyst systems. The state of the art knows several other embodiments. For example, the German patent DE 36 94 416 C2 describes homogeneously distributed precious metal mixtures. It is also possible, for example, for a plurality of precious metal foils (Pd+V+Pd) to be applied to an aluminum carrier body in a sandwich design and to be wound in spirals. Also, the European patent application EP 0,301,536 A2 describes devices, in which a palladium alloy (with a short response time to catalytic reaction) is used. The palladium alloy contains at least 80% by weight of Pd, no more than 19.9% by weight of a further metal of the 8th group of the periodic system, in particular nickel, and no more than 10% by weight of copper.
Due to the relatively great microscopic distance between the sensitive palladium centers and the poison-resistant platinum centers that are predominantly present, other catalyst mixtures cannot generate sufficient reaction rates at the low temperatures under consideration here, so that a light-off of the platinum centers cannot be achieved.
According to the prior art, to avoid deactivation, the above-described devices may also be equipped with filters or be stored in closed vessels which are opened if so required. Such additional devices, although avoiding or reducing catalyst aging, are very costly and complicated and can be designed only with difficulty in the light of other requirements, such as, for example, earthquakes.
It has become known from the German patent DE 31 43 989 C to use a recombination device which comprises as a whole a vessel and an electrical heating system. No details with regard to the design are provided by the publication.
It is accordingly an object of the invention to provide a method and a device for the early initiation of a hydrogen/oxygen reaction in a catalytic recombination or ignition device, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which render it possible for the hydrogen to be reduced passively in good time and, as a consequence, to increase the safety of the system. The device is to operate reliably under the most difficult accident conditions, in particular in catalytic ignition or recombination devices which are mounted within the safety vessel of a nuclear power plant and which operate mainly with the use of the precious metals palladium and platinum.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of initiating a hydrogen/oxygen reaction in a catalytic recombination or ignition device, in particular for a nuclear power plant. The catalytic recombination or ignition device having at least one catalyst body with a predetermined catalytic surface. The method comprises the following steps:
introducing energy into a catalyst body and permanently maintaining only a portion of a predetermined catalytic surface of the catalyst body at a temperature above ambient temperature. In a preferred embodiment, less than 5% of the surface is maintained at the elevated temperature.
There is also provided, in accordance with the invention, a combination of a catalytic recombination or ignition device having a number of catalyst bodies with a predetermined catalytic surface and a device for initiating a hydrogen/oxygen reaction in the catalytic recombination or ignition device. The combination comprises an energy feed device connected to the catalytic recombination or ignition device for introducing energy and permanently maintaining a portion of the predetermined catalytic surface, preferably less than 5% of the surface, at a temperature level above an ambient temperature.
The infeed of energy and the associated rise in temperature to a temperature level above that of the ambient temperature afford favorable reaction conditions for the hydrogen/oxygen reaction. In an incident, the sequence of the reaction is initiated in the said small portion of the surface as a consequence of the increased surface temperature. Further measures, which are defined in various dependent claims herein, ensure that the initial reaction can spread particularly quickly to the entire surface.
In accordance with an added feature of the invention, the energy is procured from a local energy store, such as a battery.
In accordance with an additional feature of the invention, the energy is introduced by permanently heating an electrical heating device or by heating a catalytically active zone.
In the preferred application of the invention, the recombination or ignition device is placed in a containment of a nuclear power plant.
In accordance with a further feature of the invention, the small portion of the catalytic surface is maintained at a temperature of more than 50xc2x0 C., preferably above 80xc2x0 C.
In accordance with again an added feature of the invention, the energy is procured from a central current supply device and, in an event of a failure of the central current supply device, a switch-over is effected to the local energy store.
In accordance with again another feature of the invention, a heat content is delivered via a heat transport element connected to a storage device (with liquid or solid energy storage material), and the storage device is heated and permanently maintained at an increased temperature, preferably above 200xc2x0 C.
The term xe2x80x9clocal energy storexe2x80x9d herein used means that the device is disposed in close vicinity of the catalytic surface. A battery, for instance, may be disposed directly at the housing of the recombination or ignition device. Thus, even in the event of an incident-related failure of the power plant current supply and in the presence of low temperatures, for example, below 10xc2x0 C., along with low hydrogen concentrations of, for example, less than 2% by volume, in the case of a highly contaminated atmosphere, in the case of moisture, etc., it becomes possible to achieve a completely passive start of the catalytic reaction. The diversion of the heat of reaction into adjacent catalytic surfaces makes it possible to activate the complete catalytic ignition or recombination device. Oxidation of the hydrogen can thus take place in good time before critical concentration limit values are reached. A decisive increase in safety in a nuclear power plant is consequently achieved.
In other words, the permanent control of the temperature of the catalytic part makes it possible, when hydrogen is released, for the heated catalyst zone to be further heated spontaneously, due to the exothermic reaction which starts immediately, and to set in motion a self-assisting catalytic reaction. The reaction, possibly assisted via metallic conductors strategically disposed in the device, is transferred into the adjacent catalyst parts. This allows a global onset of the reaction.
Due to partial heating by means of an independent energy store system and due to the combination of the measures specified, aging effects, brought about by use in the atmosphere of the nuclear power plant, such as, for example, by the sorption of hydrocarbons and welding vapors, aerosol pollution, etc., can be compensated for long operating periods of many years, without the loss, otherwise occurring after a short stand-by time, of the self-starter function which is important for safety. These operating periods may cover five years or more. Safety is therefore decisively increased, and, at the same time, costs are reduced, since a cyclic exchange of the individual catalyst bodies becomes unnecessary.
Although the invention is illustrated and described herein as embodied in method and device for initiating a hydrogen/oxygen reaction in a reactor safety vessel, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.